Description of the Prior Art
Switched capacitor circuit stages using multi-phase docks are found in a number of sampled analog data applications such as filters and signal conversion circuits, including analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). They are additionally used in emerging technologies such as image processing in digital cameras, digital camcorders, and digital high definition televisions.
An early switched capacitor implementation of the present day fast interstage amplifier with a fixed gain of two used in pipelined ADCs is disclosed in "High-Resolution Pipelined Analog-to-Digital Conversion," by Sehat Sutarja, University of California, Berkeley, 1988, pp. 40-43, which shows a 25 percent increase in speed. The switched capacitor configuration of this reference also has the unique property that the noise gain equals the signal gain, which is different from many versions of switched capacitor gain/integrator stages where the noise gain equals the signal gain plus one. See, for example, U.S. Pat. Nos. 4,354,250, 4,404,525, 4,453,258, 4,555,668 and 5,574,457.
If one uses a switched capacitor gain stage according to the Sutarja reference for a finite signal bandwidth application, one does not need an amplifier with such a large amplifier bandwidth as compared to other switched capacitor gain stages mentioned above where one needs an amplifier with a larger bandwidth thereby expending more power for the same result. The Sutarja gain stage has one capacitor that switches the bottom plate of the capacitor from the input signal to the output of the amplifier and has another capacitor that switches the bottom plate of the capacitor from the input signal to ground. Both capacitor top plates are connected to the amplifier input and a clocked grounding switch. The first capacitor's bottom plate is never switched to ground and the second capacitor is never switched to the output. The Sutarja switched capacitor gain stage has both the capacitors equal to each other and does not do any signal attenuation only signal gain.
Attenuation circuits mainly comprise of resistors with field effect transistors used as either switches or active resistive elements such as taught in U.S. Pat. No. 5,157,323. A known switched capacitor adjustable attenuator circuit, such as shown in U.S. Pat. No. 4,468,749, uses a predetermined time to discharge a capacitor to ground in order to provide attenuation. The configuration uses a feedback capacitor that is switched from across the op amp input and output on one clock cycle to both ends grounded on the other cycle.
For a programmable gain and attenuation circuit, it would be advantageous for a single switched capacitor circuit to provide both gain and attenuation in the same embodiment, and without using FETs and resistors for attenuation. Furthermore, it would be desirable to minimize the power expended by an operational amplifier in such an arrangement for gain settings.